fullerene, carbon onions, nano-diamonds), one-dimensional (carbon nanotubes,
single-walled carbon nanotubes, carbon nanofibers), and two-dimensional materials
(graphene, graphene nanoribbons, few layered graphenes) (Shandilya et al. 2018b).
Xu et al. (2004) firstly isolated carbon quantum dots from the crude soot while
preparing single-walled carbon nanotubes (Xu et al. 2004). Sun et al. (2006) isolated
carbon quantum dots from graphite powder and cement using laser ablation method
(Sun et al. 2006). Pan et al. (2018) fabricated CdS/BiOCl heterojunction via selective
deposition of CdS quantum dots on BiOCl nanosheets. Various characterization
techniques indicate the uniform dispersion of CdS quantum dots. The photocatalytic
performance was carried out against methyl orange and phenol, which shows 4.0 and
4.8 times higher efficiency of nanocomposites as compared to bare BiOCl. The
increased visible light absorption and high migration efficiency of charge carrier are
attributed for high efficiency. Due to the narrow band gap of CdS of 2.4 eV, CdS can
be easily coupled with bismuth-based photocatalyst so as to inhibit rate of recombination by promoting charge carrier separation. Earlier, BiOI, BiOCl coupled with
CdS quantum dots, and photocatalytic activity were evaluated against rhodamine B
and methyl orange (Kandi et al. 2017; Liu et al. 2014).
There are many review articles published highlighting the synthetic approaches,
properties, surface functionalization, and application of carbon quantum dots. Here,
top-up and bottom-down approaches for the synthesis of carbon quantum dots with
their advantages and disadvantages were reviewed. Then, the mechanism of some
novel photocatalysts is also proposed followed by the general discussion of
Z-scheme photocatalyst and later on the up-conversion phenomena of carbon quantum dots which are basically responsible for the higher photo-efficiency of
nanocomposites by broadening the region of solar light absorption.
3.3 Method of Preparation of Carbon Quantum
Dot-Modified Photocatalysts
Generally, there are two synthetic methods by which carbon quantum dots can be
prepared: “bottom-up” and “top-down” approaches (Roy et al. 2015; Zhu et al.
2015). The main problems taken into account while synthesizing carbon quantum
dots are agglomeration, surface functionalization, size control, and uniformity. The
top-down approach includes the fragmentation of large based precursor (graphite,
graphene, graphene oxide, carbon nanotube, carbon fiber) into carbon nanomaterial
via arc discharge, laser ablation, electrochemical approach, acid oxidizing exfoliation, and ultrasonic exfoliation (Yuan et al. 2016). However, bottom-up approach
includes hydrothermal/solvothermal method, microwave-assisted method, thermal
pyrolytic route (oil bath), reverse micelle technique, template method, and substance
oxidation (Zuo et al. 2016) (Fig. 3.4 and Table 3.1). Bottom-up approach is a
template strategy which is used to construct carbon quantum dots by using citric
86
P. Shandilya et al.
single-walled carbon nanotubes, carbon nanofibers), and two-dimensional materials
(graphene, graphene nanoribbons, few layered graphenes) (Shandilya et al. 2018b).
Xu et al. (2004) firstly isolated carbon quantum dots from the crude soot while
preparing single-walled carbon nanotubes (Xu et al. 2004). Sun et al. (2006) isolated
carbon quantum dots from graphite powder and cement using laser ablation method
(Sun et al. 2006). Pan et al. (2018) fabricated CdS/BiOCl heterojunction via selective
deposition of CdS quantum dots on BiOCl nanosheets. Various characterization
techniques indicate the uniform dispersion of CdS quantum dots. The photocatalytic
performance was carried out against methyl orange and phenol, which shows 4.0 and
4.8 times higher efficiency of nanocomposites as compared to bare BiOCl. The
increased visible light absorption and high migration efficiency of charge carrier are
attributed for high efficiency. Due to the narrow band gap of CdS of 2.4 eV, CdS can
be easily coupled with bismuth-based photocatalyst so as to inhibit rate of recombination by promoting charge carrier separation. Earlier, BiOI, BiOCl coupled with
CdS quantum dots, and photocatalytic activity were evaluated against rhodamine B
and methyl orange (Kandi et al. 2017; Liu et al. 2014).
There are many review articles published highlighting the synthetic approaches,
properties, surface functionalization, and application of carbon quantum dots. Here,
top-up and bottom-down approaches for the synthesis of carbon quantum dots with
their advantages and disadvantages were reviewed. Then, the mechanism of some
novel photocatalysts is also proposed followed by the general discussion of
Z-scheme photocatalyst and later on the up-conversion phenomena of carbon quantum dots which are basically responsible for the higher photo-efficiency of
nanocomposites by broadening the region of solar light absorption.
3.3 Method of Preparation of Carbon Quantum
Dot-Modified Photocatalysts
Generally, there are two synthetic methods by which carbon quantum dots can be
prepared: “bottom-up” and “top-down” approaches (Roy et al. 2015; Zhu et al.
2015). The main problems taken into account while synthesizing carbon quantum
dots are agglomeration, surface functionalization, size control, and uniformity. The
top-down approach includes the fragmentation of large based precursor (graphite,
graphene, graphene oxide, carbon nanotube, carbon fiber) into carbon nanomaterial
via arc discharge, laser ablation, electrochemical approach, acid oxidizing exfoliation, and ultrasonic exfoliation (Yuan et al. 2016). However, bottom-up approach
includes hydrothermal/solvothermal method, microwave-assisted method, thermal
pyrolytic route (oil bath), reverse micelle technique, template method, and substance
oxidation (Zuo et al. 2016) (Fig. 3.4 and Table 3.1). Bottom-up approach is a
template strategy which is used to construct carbon quantum dots by using citric
86
P. Shandilya et al.